Power feeding system for telephone terminal in LAN

ABSTRACT

Terminals are connected via cables in a star-shaped fashion with respect to a HUB in a LAN. Each of the cables includes therein signal lines and power feed lines. The HUB includes therein a power feed section. The power feed section and the power feed lines of each cable are connected to measure a value of current which flows in the power feed lines. If it is judged based on the measured current value that the terminal connected to the corresponding cable is a telephone terminal, the power feeding to the corresponding power feed lines is continued. On the other hand, if the terminal is judged to be a terminal other than the telephone terminal, the power feeding is stopped.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of parent application Ser. No. 10/726,511, filedDec. 4, 2003, which is a continuation of grandparent application Ser.No. 09/352,332, filed Jul. 13, 1999 (now U.S. Pat. No. 6,681,013). Thedisclosures of the parent and grandparent applications are herebyincorporated in the present application by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for feeding the power to atelephone terminal/terminals connected to a LAN (Local Area Network).

2. Description of the Related Art

In recent years, the so-called downsizing has been rapidly developed inthe information processing field. For facilitating the downsizing,technology of LAN's has been developed for achieving distributedprocessing or horizontal distributed processing using a plurality ofwork stations or personal computers. Simultaneously, technology of LANtelephone terminals adapted for the LAN environment has also beendeveloped. In this case, it is important to realize a power feedingsystem for the LAN telephone terminals.

Specifically, in transmission lines forming the LAN environment, datasignals are flowing. Thus, it is difficult to use those transmissionlines as power feed lines as in the public network. In particular, thisis almost impossible in Ethernet LAN's wherein Manchester codes havingno DC components are used as transmission codes. As is known in the art,the Manchester code is a code whose polarity is inverted at the centerof a bit sequence so as to define a logical value of 1 or 0 by adirection of the inversion. Under the circumstances, the telephoneterminals in the LAN receive the power from the commercial power sourcevia AC adapters, respectively.

In this technique, however, since the respective telephone terminalsreceive the power from the commercial power source via the AC adapters,installation of the telephone terminals is troublesome. Further, everytime an installed position of the telephone terminal is changed, anoperator is required to make the installed position thereof clearrelative to the network, thereby lacking in portability.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a powerfeeding system in a LAN which is simple in structure while being capableof automatically judging whether a connected terminal is a telephoneterminal or a terminal other than the telephone terminal, so as toautomatically feed the power to the telephone terminal.

According to one aspect of the present invention, there is provided, ina LAN having terminals connected to each other via cables extending froma line concentrator, each of the cables including therein a signal linefor delivering data signals for mutual communication between theterminals and the terminals including at least one telephone terminal, apower feeding system comprising a power feed line included in each ofthe cables; a power feed section for feeding power to the at least onetelephone terminal via the corresponding power feed line; power feedcontrol switching sections each for establishing or disablingcommunication between the power feed section and the corresponding powerfeed line; a current monitor section for detecting whether a value ofcurrent flowing in each of the power feed lines when each of the powerfeed lines is connected to the power feed section is within a presetcurrent value range which represents a state where the telephoneterminal is connected to the cable including therein the correspondingpower feed line; and a control section for controlling the power feedcontrol switching sections to connect the corresponding power feed linesto the power feed section in sequence, and for controlling each of thepower feed control switching sections to stop feeding the power via thecorresponding power feed line when the current monitor section detectsthat the value of the current flowing in the corresponding power feedline is outside the preset current value range, and to continue feedingthe power via the corresponding power feed line when the current monitorsection detects that the value of the current flowing in thecorresponding power feed line is within the preset current value range.

It may be arranged that the power feeding system further comprises alink detecting section for monitoring each of the signal lines insequence to detect whether link of the terminal connected to thecorresponding signal line is established, wherein the control sectioncontrols the corresponding power feed control switching section tocontinue feeding the power to the corresponding power feed line when thelink detecting section detects that the link of the terminal connectedto the corresponding signal line is established, while the value of thecurrent flowing in the corresponding power feed line is detected to bewithin the preset current value range.

It may be arranged that the power feeding system further comprises alink detecting section for monitoring each of the signal lines insequence to detect whether link of the terminal connected to thecorresponding signal line is established, wherein the control sectioncontrols the corresponding power feed control switching section tocontinue to stop feeding the power to the corresponding power feed linewhen the link detecting section detects that the link of the terminalconnected to the corresponding signal line is established, while thecorresponding power feed line is disconnected from the power feedsection by the corresponding power feed control switching section.

It may be arranged that the power feeding system further comprises aline state monitor table for storing given states of the power feed lineand the signal line of each of the cables, wherein the control sectioncontrols, by switching among the cables per given period, an operationof the whole power feeding system including the operations of the powerfeed control switching sections based on the line state monitor table.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinbelow, taken in conjunction with theaccompanying drawings.

In the drawings:

FIG. 1 is a block diagram of a power feeding system according to apreferred embodiment of the present invention;

FIG. 2 is a diagram showing a structure of a line state monitor tableused in the power feeding system shown in FIG. 1;

FIG. 3 is a diagram for explaining an operation of the power feedingsystem shown in FIG. 1; and

FIG. 4 is a diagram for explaining sequential operations of the powerfeeding system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, a power feeding system according to a preferred embodiment of thepresent invention will be described with reference to the accompanyingdrawings. In this embodiment, the present invention is applied to theEthernet LAN using the 10BASE-T.

FIG. 1 shows the structure of the power feeding system according to thepreferred embodiment of the present invention. In FIG. 1, the powerfeeding system comprises a HUB 1′ as a line concentrator having ports30-1 to 30-N, cables 3-1 to 3-N (N is the maximum number correspondingto the number of the ports 30-1 to 30-N, which will be also applied toother components described hereinafter), terminals 4-1 to 4-N,connectors A-1 to A-N, and connectors B-1 to B-N. Although all theterminals 4-1 to 4-N are shown to be telephone terminals in FIG. 1 forfacilitating explanation of the power feeding system, some of theterminals may be terminals other than the telephone terminals, ofcourse. The terminals 4-1 to 4-N are connected to the HUB 1′ in astar-shaped fashion via the corresponding cables 3-1 to 3-N with thecorresponding connectors A-1 to A-N and B-1 to B-N, respectively.

The HUB 1′ comprises a telephone terminal detecting section 1, powerfeed control switching sections 2-1 to 2-N and a line state monitortable 12.

The telephone terminal detecting section 1 comprises a current monitorsection 5, a power feed section 6, a timer section 7, a link detectingsection 8, a link detected state monitor section 9 and a control section11. The telephone terminal detecting section 1 judges whether each ofthe connected terminals is a telephone terminal or not, and feeds thepower when the connected terminal is judged to be the telephoneterminal.

Each of the power feed control switching sections 2-1 to 2-N is in theform of a change-over switch for establishing or disabling connectionbetween the power feed section 6 and corresponding one of pairs of powerfeed lines 3B-1 to 3B-N of the cables 3-1 to 3-N.

Each of the cables 3-1 to 3-N includes therein corresponding one ofpairs of signal lines 3A-1 to 3A-N, corresponding one of pairs of theforegoing power feed lines 3B-1 to 3B-N, and corresponding one ofgrounding lines 3C-1 to 3C-N. The cables 3-1 to 3-N connect between theHUB 1′ and the corresponding terminals 4-1 to 4-N, respectively. Thesignal lines are used for delivering data signals between thecorresponding terminals via the HUB 1′, while the power feed lines areused for feeding the DC power from the power feed section 6 to thecorresponding telephone terminals. According to the 10BASE-T, unsealedtwisted pair cables are used for the cables 3-1 to 3-N.

Each of the telephone terminals is a digital telephone adapted for theLAN. The maximum number of the telephone terminals is N (4-1 to 4-N)corresponding to the number of the ports 30-1 to 30-N of the HUB 1′ asnoted above. Each of the telephone terminals is provided withcorresponding one of DC/DC converters 10-1 to 10-N. The DC/DC converterconverts a DC voltage supplied from the HUB 1′ or the telephone terminaldetecting section 1 into a DC voltage suitable for the telephoneterminal. Accordingly, if the terminal is not the telephone terminal,the DC/DC converter is not provided.

The connectors A-1 to A-N are used for connecting the correspondingcables 3-1 to 3-N to the HUB 1′.

The connectors B-1 to B-N are used for connecting the correspondingcables 3-1 to 3-N to the terminals 4-1 to 4-N, respectively.

According to the 10 BASE-T, an 8-pin connector RJ45 (8-pin modular jack)is used for each of the connectors A-1 to A-N and B-1 to B-N. Connectpositions of 8 pins will be described later.

Now, the structure of the telephone terminal detecting section 1 will bedescribed.

The current monitor section 5 measures a value of current flowing in thepower feed lines of the corresponding cable when the telephone terminaldetecting section 1 confirms whether the connected terminal is thetelephone terminal or not.

The power feed section 6 feeds the power to the power feed lines 3B-1 to3B-N via the power feed control switching sections 2-1 to 2-N,respectively.

The timer section 7 sets a monitor time for detecting the linkestablishment of the corresponding terminal relative to the LAN, whichwill be described later. The link establishment represents a standbystate where the corresponding terminal is fully operable with the LAN.

The link detecting section 8 receives a signal from each of physicallayers 20-1 to 20-N of the network side so as to confirm whether thecorresponding telephone terminal or terminal other than the telephoneterminal has established the link relative to the LAN.

The link detected state monitor section 9 continues to monitor the linkestablishment after the detection thereof by the link detecting section8.

The control section 11 performs switching of operations, judgment of thestates and others so as to control the whole power feeding system.

The line state monitor table 12 stores states of the respective cables3-1 to 3-N under the control of the control section 11. The state ofeach cable is updated per monitor period when a change of the stateoccurs, and read out upon request.

FIG. 2 shows a structure of the line state monitor table 12. As seenfrom FIG. 2, the table is divided into 10 regions in an X-axis directionand into regions 12-1 to 12-N in a Y-axis direction. As noted above, Nrepresents the maximum number corresponding to the number of the ports30-1 to 30-N of the HUB 1′. On the other hand, the 10 regions in theX-axis direction correspond to later-described operation stages.

Now, the connect positions of the 8 pins of each of the connectors A-1to A-N will be described. The foregoing 8-pin connector RJ45 has No. 1pin to No. 8 pin. No. 3 pin and No. 6 pin are connected to atransmission line of the network side, while No. 1 pin and No. 2 pin areconnected to a reception line of the network side. Among No. 4 pin, No.5 pin, No. 7 pin and No. 8 pin, two of them (No. 4 pin and No. 5 pin inthis embodiment) are used for power feeding, while the remaining two(No. 7 pin and No. 8 pin in this embodiment) are open-circuit orgrounded.

Similarly, the connect positions of the 8 pins of each of the connectorsB-1 to B-N will be described. The foregoing 8-pin connector RJ45 has No.1 pin to No. 8 pin. No. 1 pin and No. 2 pin are connected to atransmission line of the terminal side, while No. 3 pin and No. 6 pinare connected to a reception line of the terminal side. Among No. 4 pin,No. 5 pin, No. 7 pin and No. 8 pin, two of them (No. 4 pin and No. 5 pinin this embodiment) are used for power feeding, while the remaining two(No. 7 pin and No. 8 pin in this embodiment) are grounded to theterminal.

Further, No. 1 pin and No. 2 pin of each of the connectors A-1 to A-Nare connected to No. 3 pin and No. 6 pin of corresponding one of theconnectors B-1 to B-N, No. 3 pin and No. 6 pin of each of the connectorsA-1 to A-N are connected to No. 1 pin and No. 2 pin of corresponding oneof the connectors B-1 to B-N, and No. 4 pin, No. 5 pin, No. 7 pin andNo. 8 pin of each of the connectors A-1 to A-N are connected to No. 4pin, No. 5 pin, No. 7 pin and No. 8 pin of corresponding one of theconnectors B-1 to B-N.

Now, an operation of the foregoing power feeding system will bedescribed.

Among the ports 30-1 to 30-N of the HUB 1′, an operation only at theport 30-1 will be described with reference to FIG. 3 for brevity ofdescription. As appreciated, an operation at each of the subsequentports 30-2 to 30-N is the same as that at the port 30-1 represented bysteps S-2 to S-9 b shown in FIG. 3.

At step S-1, an operator turns on a power switch of the power feedingsystem. In this embodiment, a start switch of the LAN also works as thepower switch of the power feeding system. Accordingly, the power feedingsystem is activated, and the routine proceeds to step S-2.

At step S-2, under the control of the control section 11, the linkdetecting section 8 monitors the network for a time T1 set by the timersection 7. If the link establishment of the terminal 4-1 is detected,the terminal 4-1 is judged to be a terminal, such as a work station or apersonal computer, other than a telephone terminal. The reason is thatsince the terminal 4-1 is not yet fed with the power via the power feedlines 3B-1 at this time point, the terminal 4-1 is not detected if theterminal 4-1 is the telephone terminal. Subsequently, the routineproceeds to step S-2 a where a logical value “1” is written into aregion (S-2 a) of a line 1 state 12-1 (corresponding to a state of thecable 3-1) in the line state monitor table 12, and then proceeds to stepS-3. On the other hand, if the link establishment is not detected atstep S-2, the terminal 4-1 may be a terminal other than a telephoneterminal which is in a power-off state, or a telephone terminal awaitingthe power feeding. It may also be considered that no terminal isconnected. In this case, the routine proceeds to step S-2 b where alogical value “1” is written into a region (S-2 b) of the line 1 state12-1, and then proceeds to step S-4.

At step S-3, under the control of the control section 11, the linkdetected state monitor section 9 continues to monitor the link detectedstate in place of the link detecting section 8. If this link detectedstate continues, the network is in a normal operation state with respectto the port 30-1. In this case, the routine proceeds to step S-3 a wherea logical value “1” is written into a region (S-3 a) of the line 1 state12-1. On the other hand, if the state is changed to non-detection of thelink establishment, the routine proceeds to step S-3 b where a logicalvalue “1” is written into a region (S-3 b) of the line 1 state 12-1, andthen returns to step S-2 for executing step S-2 and subsequent steps.

On the other hand, if the link establishment is not detected at stepS-2, the control section 11 executes a terminal check in the followingmanner:

Specifically, at step S-4, the control section 11 controls the powerfeed control switching section 2-1 to establish connection between No. 4and No. 5 pins of the connector A-1 and the power feed section 6 so thatthe DC voltage is applied to the power feed lines 3B-1 of the cable 3-1.Then, the routine proceeds to step S-5.

At step S-5, under the control of the control section 11, the currentmonitor section 5 measures a value of current flowing in No. 4 and No. 5pins of the connector A-1 and judges whether the measured current valueis within a preset current value range. The preset current value rangeis a given range across an adequate current value obtained by connectinga telephone terminal. According to the type of the connected telephoneterminal, the upper and lower limits of the current value range aredetermined. The current value range may be determined experientially.

Therefore, if the foregoing measured current value is within the presetcurrent value range, probability is high that the terminal 4-1 is atelephone terminal. In this case, the voltage continues to be appliedacross No. 4 and No. 5 pins of the connector A-1. Subsequently, theroutine proceeds to step S-5 a where a logical value “1” is written intoa region (S-5 a) of the line 1 state 12-1, and then proceeds to stepS-6. On the other hand, if the foregoing measured current value isoutside the preset current value range, probability is high that theterminal 4-1 is a terminal other than a telephone terminal. The reasonis that if the terminal 4-1 is not a telephone terminal, No. 4 and No. 5pins of the connector A-1 are grounded to the terminal 4-1 oropen-circuit at the side of the connector B-1. Specifically, whengrounded, the foregoing measured current value becomes greater than thepreset current value range. In this case, however, it is designed thatcurrent higher than an upper limit set in view of safety is preventedfrom flowing. On the other hand, when open-circuit, almost no currentflows. In this case, the routine proceeds to step S-5 b where a logicalvalue “1” is written into a region (S-5 b) of the line 1 state 12-1, andthen proceeds to step S-8 where the voltage feeding is stopped.

At step S-6, the link detecting section 8 monitors the network for atime T2 set by the timer section 7. If the link establishment of theterminal 4-1 is detected, the routine proceeds to step S-6 a where alogical value “1” is written into a region (S-6 a) of the line 1 state12-1, and then proceeds to step S-3 where, as described above, the linkdetected state monitor section 9 continues to monitor the link detectedstate in place of the link detecting section 8. In this case, theterminal 4-1 is a telephone terminal. As long as this link detectedstate continues, the network is in a normal operation state with respectto the port 30-1, and the current continues to be supplied to thetelephone terminal 4-1 within the preset current value range. On theother hand, if the link detected state is not continued, the routinereturns to step S-2 for executing step S-2 and subsequent steps.

On the other hand, if the link establishment of the terminal 4-1 is notdetected at step S-6, probability is high that the terminal 4-1 is aterminal other than a telephone terminal which is in a power-off state.The reason is that since the voltage continues to be applied to thepower feed lines 3B-1, if the terminal 4-1 is a telephone terminal, thelink establishment thereof should be detected. In this case, the routineproceeds to step S-6 b where a logical value “1” is written into aregion (S-6 b) of the line 1 state 12-1, and then proceeds to step S-7.

At step S-7, the voltage feeding to No. 4 and No. 5 pins of theconnector A-1 is stopped. Subsequently, the routine returns to step S-4where the terminal check is started again. If the terminal 4-1 is atelephone terminal, a loop of steps S-4, S-5, S-6 and S-7 is repeateduntil the link establishment is detected at step S-6. On the other hand,if the terminal 4-1 is a terminal other than a telephone terminal, theforegoing loop is repeated until the routine proceeds from step S-5 tostep S-8. Since the value of current flowing in No. 4 and No. 5 pins ofthe connector A-1 during repetition of the foregoing loop is within thepreset current value range, even if the loop repetition extends over along time, no particular problem is raised.

Explanation will be made of the case wherein the routine proceeds fromstep S-5 to step S-8. As described at step S-5, if the foregoingmeasured current value is outside the preset current value range,probability is high that the terminal 4-1 is a terminal other than atelephone terminal. In this case, the routine proceeds to step S-8 wherethe voltage feeding to No. 4 and No. 5 pins of the connector A-1 isstopped, and then proceeds to step S-9.

At step S-9, the link detecting section 8 monitors the network for atime T3 set by the timer section 7. If the link establishment of theterminal 4-1 is detected, the routine proceeds to step S-9 a where alogical value “1” is written into a region (S-9 a) of the line 1 state12-1, and then proceeds to step S-3 where, as described above, the linkdetected state monitor section 9 continues to monitor the link detectedstate in place of the link detecting section 8. In this case, theterminal 4-1 is a terminal other than a telephone terminal. As long asthis link detected state continues, the network is in a normal operationstate with respect to the port 30-1. Since the voltage feeding isstopped at step S-8, no current flows in the power feed lines 3B-1.

On the other hand, if the link establishment is not detected at stepS-9, the routine proceeds to step S-9 b where a logical value “1” iswritten into a region (S-9 b) of the line 1 state 12-1, and then returnsto step S-4 to repeat a loop of steps S-4, S-5, S-8 and S-9 until thelink establishment is detected at step S-9, i.e. until the terminal 4-1is turned on. Alternatively, if no terminal is connected, the loop isrepeated until a telephone terminal or a terminal other than thetelephone terminal which is in a power-on state is connected. Since thevalue of current flowing in No. 4 and No. 5 pins of the connector A-1during repetition of this loop is not higher than the upper limit set inview of safety, even if the loop repetition extends over a long time, noparticular problem is raised.

FIG. 4 is a diagram showing a state wherein the telephone terminaldetecting section 1 controls the port 30-1 through the port 30-N insequence. In FIG. 4, K-1 to K-N represent clock pulses for starting thecontrol of the corresponding ports 30-1 to 30-N, respectively. The clockperiod is set to T0 by the timer section 7.

In response to the clock pulse K-1, the control section 11 reads out theline 1 state 12-1 from the line state monitor table 12. The read-outdata are previous data (T0×N prior data) and given by logical values “1”and “0” as described above.

It is assumed that the control section 11 reads out “1” from the region(S-3 a) and “0” from the region (S-3 b) of the line 1 state 12-1. Inthis case, the control section 11 executes step S-3 and judges the port30-1 to be in an effective state as long as the link detected state iscontinued, so that the control section 11 maintains the current statewith respect to the port 30-1. On the other hand, if it is assumed thatthe control section 11 reads out “1” from the region (S-3 b) of the line1 state 12-1, the control section 11 judges that the state of the cable3-1 has been changed, resets all the data of the line 1 state 12-1 tozero, and returns to step S-2 to execute step S-2 and subsequent steps.Further, it is assumed that the control section 11 reads out “0” fromthe region (S-3 a) and “0” from the region (S-3 b) of the line 1 state12-1. In this case, the control section 11 judges that the terminalcheck is finished in the middle of one of the foregoing loops having “1”in the region (S-6 b) or (S-9 b), and returns to step S-4 through thatloop so as to restart the terminal check.

The control section 11 executes the operation in the foregoing mannerand continues execution thereof up to receipt of the clock pulse K-2. Inresponse to the clock pulse K-2, the control section 11 switches fromthe port 30-1 to the port 30-2. Simultaneously, the control section 11reads out the line 2 state 12-2 from the line state monitor table 12 andstarts the control of the port 30-2. Thereafter, the control section 11controls the subsequent ports 30-3 through 30-N similarly in response tothe clock pulses K-3 through K-N.

More accurate cable controls may be realized by using other data on theline state monitor table 12 as well.

According to the foregoing preferred embodiment of the presentinvention, since it is not necessary for the respective telephoneterminals to receive the power from the commercial power source via theAC adapters, the installation of the telephone terminals is facilitated.Further, since it is not necessary for the operator to make theinstalled position of the telephone terminal clear relative to thenetwork every time the installed position thereof is changed, theportability is improved. In addition, since the voltage applied to thecable connecting between the HUB and the telephone terminal is low, itis possible to use the unsealed twisted pair cable as the cable and themodular jacks as the connectors at both ends thereof, so that theinstallation of the telephone terminal is further facilitated andreduced in cost. As appreciated, since it is sufficient only to connectthe modular jack to the HUB upon connecting the telephone terminal tothe LAN, the portability is further improved. Moreover, since N cablescan be controlled only by one telephone terminal detecting section, thesystem is simple in structure.

While the present invention has been described in terms of the preferredembodiment, the invention is not to be limited thereto, but can beembodied in various ways without departing from the principle of theinvention as defined in the appended claims. For example, the presentinvention is not limited to the Ethernet LAN using the 10BASE-T, but isalso applicable to all star-shaped LAN's wherein terminals are connectedin a star-shaped fashion by a HUB or another line concentrator replacingit.

1. An improved LAN having terminals connected to a line concentrator viasignal lines for carrying signals between the terminals and the lineconcentrator, wherein the improvement comprises: power feed linesconnected to the terminals; and a power feeding system that includes apower feed section for selectively feeding power to the power feedlines, a current monitor section for detecting whether current isflowing to each terminal and, if so, whether the current has a valuewithin a preset current range which indicates that the respectiveterminal receiving the current is a telephone terminal, a controlsection for controlling the power feed section to stop feeding power toparticular terminals if the current monitor section detects that eachcurrent flowing to those terminals has a value that is outside thepreset current value range, and to continue feeding current toparticular terminals if the current monitor section detects that eachcurrent flowing to those terminals has a value within the preset currentrange, and a line state monitor table for storing given states of thepower feed lines and the signal lines, wherein the control sectioncontrols operation of the power feeding system based on the line statemonitor table.
 2. The improved LAN of claim 1, wherein the power feedingsystem further comprises: a link detecting section for sequentiallymonitoring the signal lines to detect whether links to correspondingones of the terminals exist, wherein the control section controls thepower feeding section to continue feeding current to terminals to whichlinks exist if each current to the terminals to which links exist isdetected by the current monitor section to be within the preset currentrange.
 3. The improved LAN of claim 2, wherein the control sectionswitches periodically among the power feed and signal lines of theterminals to control operation of the power feeding system based on theline state monitor table.
 4. The improved LAN of claim 1, wherein thepower feeding system further comprises: a link detecting section forsequentially monitoring the signal lines to detect whether links tocorresponding ones of the terminals exist, wherein if terminals to whichlinks exist are not being fed current by the power feeding section, thecontrol section controls the power feed section to continue not feedingthem power.
 5. The improved LAN of claim 4, wherein the control sectionswitches periodically among the power feed and signal lines of theterminals to control operation of the power feeding system based on theline state monitor table.
 6. The improved LAN of claim 1, wherein thecontrol section switches periodically among the power feed and signallines of the terminals to control operation of the power feeding systembased on the line state monitor table.
 7. The improved LAN of claim 1,further comprising cables connected to the terminals, the cablesincluding the signal lines and the power feed lines.
 8. A power feedingsystem for use with a LAN having a line concentrator that is connectedto terminals by cables, the cables including signal lines for carryingsignals between the terminals and the line concentrator, said powerfeeding system comprising: power feed lines in the cables in addition tothe signal lines; and a power feeding unit that includes a power feedsection for selectively feeding power to the power feed lines, a monitorsection for detecting whether power is being supplied to each terminaland, if so, whether the power has a value within a preset range whichindicates that the respective terminal receiving the power is atelephone terminal, a control section for controlling the power feedsection to stop feeding power to particular terminals if the monitorsection detects that each power which is being supplied to thoseterminals has a value that is outside the preset range, and to continuefeeding power to particular terminals if the monitor section detectsthat each power which is being supplied to those terminals has a valuewithin the preset range, and a line state monitor table for storinggiven states of the power feed lines and the signal lines, wherein thecontrol section controls operation of the power feeding unit based onthe line state monitor table.
 9. The power feeding system of claim 8,wherein the power feeding unit comprises: a link detecting section forsequentially monitoring the signal lines to detect whether links tocorresponding ones of the terminals exist, wherein the control sectioncontrols the power feeding section to continue feeding power toterminals to which links exist if each power which is being fed to theterminals to which links exist is detected by the monitor section bewithin the preset range.
 10. The power feeding system of claim 9,wherein the control section switches periodically among the power feedand signal lines of the terminals to control operation of the powerfeeding system based on the line state monitor table.
 11. The powerfeeding system of claim 8, wherein the power feeding unit comprises: alink detecting section for sequentially monitoring the signal lines todetect whether links to corresponding ones of the terminals exist,wherein if terminals to which links exist are not being fed power by thepower feeding section, the control section controls the power feedsection to continue not feeding them power.
 12. The power feeding systemof claim 11, wherein the control section switches periodically among thepower feed and signal lines of the terminals to control operation of thepower feeding system based on the line state monitor table.
 13. Thepower feeding system of claim 8, wherein the control section switchesperiodically among the power feed and signal lines of the terminals tocontrol operation of the power feeding system based on the line statemonitor table.
 14. A power feeding system for use with a LAN having aline concentrator that is connected to terminals by cables, the cablesincluding signal lines for carrying signals between the terminals andthe line concentrator, said power feeding system comprising: power feedlines in the cables in addition to the signal lines; and a power feedingunit that includes a power feed section for selectively feeding power tothe power feed lines, a monitor section for detecting whether power isbeing supplied to each terminal and, if so, whether the power has avalue within a preset range which indicates that the respective terminalreceiving the power is a telephone terminal, a control section forcontrolling the power feed section to apply a voltage to the terminals,to stop applying voltage to particular terminals if the monitor sectiondetects that each power which is being supplied to those terminals has avalue that is outside the preset range, and to continue applying voltageto particular terminals if the monitor section detects that each powerwhich is being supplied to those terminals has a value within the presetrange, and a line state monitor table for storing given states of thepower feed lines and the signal lines, wherein the control sectioncontrols operation of the power feeding system based on the line statemonitor table.
 15. The power feeding system of claim 14, wherein thepower feeding unit comprises: a link detecting section for sequentiallymonitoring the signal lines to detect whether links to correspondingones of the terminals exist, wherein the control section controls thepower feeding section to continue applying voltage to terminals to whichlinks exist if each power which is being fed to the terminals to whichlinks exist is detected by the monitor section be within the presetrange.
 16. The power feeding system of claim 15, wherein the controlsection switches periodically among the power feed and signal lines ofthe terminals to control operation of the power feeding system based onthe line state monitor table.
 17. The power feeding system of claim 14,wherein the power feeding unit comprises: a link detecting section forsequentially monitoring the signal lines to detect whether links tocorresponding ones of the terminals exist, wherein if terminals to whichlinks exist are not being fed power by the power feeding section, thecontrol section controls the power feed section to continue not feedingthem power.
 18. The power feeding system of claim 17, wherein thecontrol section switches periodically among the power feed and signallines of the terminals to control operation of the power feeding systembased on the line state monitor table.
 19. The power feeding system ofclaim 14, wherein the control section switches periodically among thepower feed and signal lines of the terminals to control operation of thepower feeding system based on the line state monitor table.